This invention relates generally to molded case circuit breakers of the type which have blow off contacts and more particularly to molded case circuit breakers which house individual poles of a circuit breaker.
Circuit breakers are widely used to open and close circuits to control current flow. One type of current interrupting mechanism known as a blow off mechanism is commonly used to handle massive overcurrent conditions and to instantaneously open during the first milliseconds that a massive overcurrent condition exists. This type of current limiting circuit breaker employs a high speed mechanism to rapidly separate the contacts upon occurrence of a fault condition to draw an arc between the contacts, allowing the voltage drop across the arc to limit the current flow. The electrodynamic force produced by the current flow through the circuit interrupter is used to rapidly drive the contacts apart and force the arc into an arc extinction device. When the blow open contacts open due to a fault, tremendous gas pressure is generated which severely stresses the circuit breaker case. Because the gas pressure is high and the case walls are relatively thin, the case is susceptible to breaking due to the pressure and the stresses. Accordingly, it can be appreciated that it would be highly desirable to provide a case which resists breaking due to stresses caused by the gas pressure during operation.
The molded casings for circuit breakers are typically constructed of a glass polyester or other material. In the past, the circuit breaker molded case or housing has been basically a two-piece molding. The bottom portion was constructed to house the components while the top portion was just a cover. At one time, little attention was paid to the physical size of the circuit breaker and, as a result, the housings and the circuit breaker itself were large and bulky. Of course, these large, bulky constructions had the requisite strength required to withstand the gas pressures generated during severe operation. However, with the modern trend toward miniaturization of components to maximize the space available, circuit breakers have been undergoing a reduction in size. This size reduction for the circuit breakers enables the component into which the molded case circuit breaker is placed to also undergo a size reduction. These size reductions are very important because they save valuable space and reduce overall costs. But, with the reduction in size has come the tendency to use thinner housings which are more susceptible to breaking and failure than their thicker predecessors. Thus, it can be appreciated that it would be highly desirable to provide a compact circuit breaker housing which has the requisite strength to resist breaking and failure during operation.
One way to have a compact circuit breaker arrangement is to use metal to reinforce the plastic housing; in such a construction the internal components of the circuit breaker poles are mounted on or housed in metal which then attaches to the molded case. Naturally, this is more expensive than mounting the parts directly to the molded housing without the use of additional metal. Also, fabricating metal components is more costly than molding housing parts using the modern materials such as glass reinforced polyester. Accordingly, it would be highly desirable to provide a strong molded case circuit breaker which uses a minimum amount of metal for the requisite strength.
In addition to the molded case being easier and cheaper to fabricate than a metal reinforced case, it is more advantageous to mold cases for individual poles rather than to house multiple poles in a single housing. Housing individual poles in their own housing also has the advantage that certain poles may be interchangeable. Accordingly, it can be appreciated that it would be highly desirable to have a molded case housing for a circuit breaker wherein each pole has its own molded case housing.
Accordingly, it is an object of the present invention to provide a molded case circuit breaker which has a reinforced molded case housing.
Another object of the invention is to provide a molded case housing which houses individual poles of a circuit breaker.
Another object of the present invention is to provide a molded case circuit breaker housing which is strong enough to withstand the tremendous gas pressure generated during operation of the circuit breaker.